Implement HttpClient.DefaultProxy property (dotnet/corefx#37333)

Implement HttpClient.DefaultProxy property (dotnet/corefx#37333)

This PR implements the new static HttpClient.DefaultProxy property which was approved
during API review.

Modify the SystemProxyInfo.ConstructSystemProxy method to a Singleton.

Modify SocketsHttpHandler to use the HttpClient.DefaultProxy property.

Rename the HttpSystemProxy class to HttpWindowsProxy.

Add some HttpClient tests for the new property.

Closes dotnet/corefx#36553

Commit migrated from https://github.com/dotnet/corefx/commit/c661db6ce2da91033671c0000afc185de1ad3fe9